Overdrive techniques for improving the response times of displays such as liquid crystal displays are known. According to such techniques, when a change in the optical state of a pixel is required, a voltage greater than that for producing the desired new state is initially applied to the pixel. This causes the pixel state to change more rapidly than would have been the case if the voltage corresponding to the desired state had been initially applied. After one or more frame periods, the voltage is reduced to that actually required for the desired optical state of the pixel.
An example of this is illustrated in FIGS. 1 and 2 of the accompanying drawings. FIG. 1 is a graph of the response of a pixel of a typical liquid crystal display (LCD) to a desired change in state, together with the ideal response. Before time frame zero, the data for the pixel represented zero corresponding, in the case of a normally black display, to a black pixel of substantially zero luminance. At time zero, the pixel data is changed to 64 corresponding to a desired grey-scale luminance illustrated by the “ideal response” curve and the “destination”. The ideal response would be for the pixel immediately to assume the luminance corresponding to the data 64 but, because of the finite and relatively slow liquid crystal response time, the pixel takes more than four frame periods to achieve the desired luminance.
FIG. 2 of the accompanying drawings illustrates the effect of applying overdrive. At time zero, although the desired luminance corresponds to pixel data 64, an overdrive level corresponding to an “overdrive target” of 120 is actually applied to the pixel. This causes the pixel to respond much more rapidly such that its luminance rises to a level much closer to the desired value. For subsequent frames, assuming that the desired pixel state is not changed, the “destination” value of 64 is applied as pixel data and the pixel achieves the desired luminance within approximately two display frame periods. The pixel therefore responds much more quickly and reaches the region of the target value, in this example, in less than half the time required without overdrive.
U.S. Pat. No. 6,747,621 discloses a liquid crystal display which performs this type of overdrive. The display comprises a frame memory which is used for delaying the current display data supplied to the liquid crystal device by one display (frame) period. The display also comprises a reference table memory which is addressed by the current pixel value and the pixel value from the previous frame delayed by the frame memory. The memory contains a look-up table down-loaded from a non-volatile memory for selecting the pixel data or level actually supplied to the liquid crystal device as a function of the current and previous values of the pixel. In the case of 8 bit pixel data, the frame memory has to be large enough to hold a complete frame of display data and the look-up table memory requires 256×256×8 bit capacity in order to perform the correct driving of the display device. Further, a substantial amount of non-volatile memory is required in order to store the look-up table with the overdrive information for each possible pixel transition so as to allow for optimisation during development or assembly.
It may also be necessary to operate the display at a higher than normal supply voltage in order for the appropriate overdrive voltages to be available throughout the possible range of transitions in grey-scale. The use of a higher supply voltage results in higher power consumption of the display and this is generally undesirable but particularly so for mobile application which rely on batteries for their power supply.
U.S. Pat. No. 6,937,232 discloses a similar arrangement but with the “overdrive circuitry” transferred from a display unit to an external unit or personal computer where additional memory is available in order to reduce the cost of the display.
U.S. Pat. No. 6,930,663 discloses a technique for suppressing colour shift at sharp image boundaries by increasing the response time of some pixel colours in order to match the response of the slowest colour pixel when overdrive is applied.
WO 2005/101364 discloses a liquid crystal display which provides overdrive based on the current pixel value and the pixel value in the previous frame. The display also deals with “sticky pixels” which were in a state from which their response time is too slow for conventional overdrive. When such sticky pixels are detected in the previous two frames, a “pretilt” voltage is applied in the previous frame before applying overdrive in the current frame.
US 2004/0090407 discloses a liquid crystal display in which overdrive is provided based on the current value and the pixel value in the previous frame. A flag is set according to whether the pixel value has changed and is used to control overdrive.